1. Field of the Invention
The present invention relates to a method and apparatus for reproducing an original gray scale or half-tone image on a reproduction medium.
Both alpha-numeric and pictorial images may be reproduced by this method and apparatus. Moreover, the image may be reproduced at the site of the original image or a site remote from the original image using known data transmission techniques.
2. Description of the Prior Art
Various methods and apparatus have been proposed in the past for reproducing gray scale or half-tone images by either darkening or not darkening areas on a reproduction medium. For example, U.S. Pat. No. 3,604,846 (Behane et al.) discloses a system in which original images are divided into discrete picture elements or pels, each of which is scanned to determine its gray level. Each pel is reproduced as a matrix of, for example, nine locations, some or all of which are darkened by an ink jet printing mechanism that prints dots, each having maximum density within the capability of the printing mechanism. Variations in the gray level of the pel are approximated by printing greater or fewer dots in a given matrix area, the entire image being comprised of a large number of matrix areas, each corresponding to one pel. A given original pel gray level produces a reproduction matrix always having the same number of dots. Further, original pels of increasing gray levels are reproduced by printed matrices having increasing numbers of dots printed in regular patterns beginning in the upper left corner thereof and filling toward the lower right corner. Thus a pel having a gray level requiring six dots to be printed will always be reproduced as a matrix with six dots clustered toward the upper left corner.
U.S. Pat. No. 3,373,437 (Sweet et al.) discloses a fluid droplet printing apparatus in which half-tones are reproduced by varying the number of drops deposited in a given dot area to vary the density thereof.
U.S. Pat. No. 3,977,007 (Berry et al.) discloses a gray tone generation system which reproduces pels having differing shades of gray by depositing, at predetermined dot locations in a matrix, a number of dots of ink calculated to represent the shade being recorded. The Berry system is similar to that disclosed in the Behane et al. Patent. However, the matrix is not filled in a regular order to represent gradations of gray level. Instead, the matrix locations are filled in an ordered, though random, pattern to approximate the gray level. That is, if the gray level being reproduced requires printing of eight dots in a sixteen location matrix, the dots are always printed in the same locations. However, the printed locations are not clustered or otherwise arranged in a regular manner as they are in the Behane et al. matrix.
Methods and apparatus for generating half-tone images such as those disclosed in the patents reviewed above have certain drawbacks. First, because original image pels are reproduced by matrices of ordered patterns of dots, the resolution of the image is necessarily limited. Second, because each pel is represented by a matrix of many dots, relatively large amounts of data are needed to reproduce a pel gray level that could otherwise be expressed with relatively little data. For example, if it is assumed that a pel has an arbitrary gray level of from (1) to (16), (1) representing the lightest level, (16) representing the darkest level, the pel gray level may be digitally represented by only four bits of data. However, if the pel is reproduced on a matrix of four-by-four dot locations, each of which may or may not be darkened in a random manner, 16 bits of data are necessary to digitally represent the matrix. Moreover, such systems are limited by the mechanical capability of the printing mechanism which they use and, in particular, by the minimum dot size that may be printed. For example, if the ratio of the area of the original image to the reproduced image is to be one-to-one, if the system's printer can print a maximum of 200 dots per inch, and if each pel is represented by a four-by-four dot matrix, then the maximum number of pels which may usefully be scanned per inch is 50. Yet, this limitation further reduces the resolving power of the system which may be optimized by dividing the original image into as many pels as possible within the system capability.
These drawbacks may be traced to the fact that prior art systems such as those described only consider pel gray levels and do not consider gray level distribution. That is, these systems only consider the first moment of the pel function of both gray level and distribution. The method and apparatus of the present invention are intended to remedy these drawbacks.